[PATCH] xtensa: Make instruction cost estimation for size more accurate

[Takayuki 'January June' Suwa via Gcc-patches]

Until now, we applied COSTS_N_INSNS() (multiplying by 4) after dividing
the instruction length by 3, so we couldn't express the difference less
than modulo 3 in insn cost for size (e.g. 11 Bytes and 12 bytes cost the
same).

This patch fixes that.

;; 2 bytes
addi.n  a2, a2, -1      ; cost 3

;; 3 bytes
addmi   a2, a2, 1024    ; cost 4

;; 4 bytes
movi.n  a3, 80          ; cost 5
bnez.n  a2, a3, .L4

;; 5 bytes
srli    a2, a3, 1       ; cost 7
add.n   a2, a2, a2

;; 6 bytes
ssai    8               ; cost 8
src     a4, a2, a3

:: 3 + 4 bytes
l32r    a2, .L5         ; cost 9

;; 11 bytes             ; cost 15
;; 12 bytes             ; cost 16

gcc/ChangeLog:

        * config/xtensa/xtensa.cc (xtensa_insn_cost):
        Let insn cost for size be obtained by applying COSTS_N_INSNS()
        to instruction length and then dividing by 3.
---
 gcc/config/xtensa/xtensa.cc | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/gcc/config/xtensa/xtensa.cc b/gcc/config/xtensa/xtensa.cc
index a1f184950ae..6cf6b35399a 100644
--- a/gcc/config/xtensa/xtensa.cc
+++ b/gcc/config/xtensa/xtensa.cc
@@ -4519,13 +4519,15 @@ xtensa_insn_cost (rtx_insn *insn, bool speed)
 {
   if (!(recog_memoized (insn) < 0))
     {
-      int len = get_attr_length (insn), n = (len + 2) / 3;
+      int len = get_attr_length (insn);
 
       if (len == 0)
        return COSTS_N_INSNS (0);
 
       if (speed)  /* For speed cost.  */
        {
+         int n = (len + 2) / 3;
+
          /* "L32R" may be particular slow (implementation-dependent).  */
          if (xtensa_is_insn_L32R_p (insn))
            return COSTS_N_INSNS (1 + xtensa_extra_l32r_costs);
@@ -4572,10 +4574,11 @@ xtensa_insn_cost (rtx_insn *insn, bool speed)
            {
              /* "L32R" itself plus constant in litpool.  */
              if (xtensa_is_insn_L32R_p (insn))
-               return COSTS_N_INSNS (2) + 1;
+               len = 3 + 4;
 
-             /* Consider ".n" short instructions.  */
-             return COSTS_N_INSNS (n) - (n * 3 - len);
+             /* Consider fractional instruction length (for example, ".n"
+                short instructions or "L32R" litpool constants.  */
+             return (COSTS_N_INSNS (len) + 1) / 3;
            }
        }
     }
-- 
2.30.2